1. Field of the Invention
This invention relates to a method of producing the radioisotope of iodine, I-131, from a tellurium oxide intermediate irradiated by a neutron flux in a nuclear reactor.
2. Description of the Prior Art
The radioisotope of iodine, I-131 (hereinafter referred to as I-131), has so far been produced from metallic tellurium or tellurium compounds irradiated by a neutron flux. This is, when tellurium is exposed to a neutron flux from a nuclear reactor or other neutron source for a suitable period of time, the isotope of tellurium, Te-130, which is contained in tellurium in the proportion of 34.49%, is converted to radioactive tellurium, Te-131, and this radioactive isotope is converted to I-131 with spontaneous emission of .beta.-rays. The technical difficulty in the application of the above method resides in the complicated separation process in which pure I-131 in the useful form is to be separated from the tellurium starting material.
Either a wet method or dry method has so far been used for the separation process. With the wet method metallic tellurium powder was, at first, used as a starting material. This material is irradiated with a neutron beam in a nuclear reactor, and then dissolved in a mixture comprising conc. sulfuric acid and chromic acid anhydride. A very vigorous reaction takes place upon the dissolution, and the tellurium is converted to telluric acid and the iodine (I-131) to iodic acid. By adding oxalic acid to the above solution, the I-131 is reduced to elemental iodine and is separated by means of distillation. The I-131 thus obtained is heavily contaminated by the reagents which are used in a large amount in the above process and, therefore, a further purification process is required. The above problem was partly overcome by using telluric acid, which is soluble in water or mineral acids, as a starting material. R. Constant, Journal of Inorganic and Nuclear Chemistry, Vol. 7, pp. 133-139, (1958). By using telluric acid, the separation process is made fairly simple. However, the stability of telluric acid is low at high temperature. This brings about a large disadvantage in irradiation by a nuclear reactor; namely, there is danger that the material will decompose during irradiation and the container for irradiation may burst owing to the increase of internal pressure resulting from generation of gases accompanying the decomposition. On the other hand, with the dry method, which was reported by K. Taugbol and J. B. Dahl in JENER REPORT No. 52, I-131 is separated from the irradiated powder of tellurium dioxide by means of dry distillation at a temperature ranging from 680.degree. - 700.degree. C in an air or oxygen stream. This process does not require complicated dissolution and separation processes which require a large amount of reagents, and the apparatus and the operation are simplified. In addition, with the dry method only I-131 is distilled and the radioactivity of I-131 is not diluted with water, whereas with the wet method water is also distilled with I-131. However, this method has an unavoidable defect in that the final product, distilled I-131, is contaminated by the tellurium volatilized together with I-131. The distillation temperature should be high in order to separate I-131 from said powder efficiently; but at the same time, this increases the volatilization of tellurium, and thereby the purity of the distilled I-131 is lowered.
The present inventors previously found that the above problems could be solved by using tellurium trioxide as a starting material, and completed a novel and excellent method of producing I-131. [See Japanese application No. 23537/1970 and U.S. application Ser. No. 125,337, filed Mar. 17, 1971 now U.S. Pat. No. 3,772,146]. In this method the properties of tellurium trioxide are utilized which comprise stability at temperatures up to about 400.degree. C (i.e. tellurium trioxide does not substantially change chemically), stability under irradiation by the nuclear reactors usually used for research and the thermal property that tellurium trioxide readily releases I-131 included in the crystal lattice accompanying the decomposition that begins at about 400.degree. C. The temperature characteristics of tellurium trioxide, when used as a starting material, result in large advantages over telluric acid which decomposes at about 110.degree. C. Further, the advantages of this method are that I-131 can be recovered with high recovery ratio in a short time at a much lower temperature than is required for the process using tellurium dioxide as a starting material (actually, it is suitable to heat irradiated tellurium trioxide at 450.degree. C in order to promote the decomposition and the accompanying release of I-131 from said material), and moreover, the low distillation temperature eliminates worry about the volatilization of tellurium, which was a defect of conventional methods. However, it is feared that the temperature of such material may be raised higher than about 400.degree. C when it is irradiated with a high density neutron flux in the nuclear reactor used for research in order to produce a large amount of I-131. Accordingly, there are some practical problems in using a large nuclear reactor for the irradiation of tellurium trioxide.